HONOLULU, Aug 6 (Reuters) - Hawaiians braced for a one-two punch from a pair of major storms headed their way on Wednesday, as Hurricane Iselle bore down on the islands packing high winds and heavy surf and Julio, tracking right behind, was upgraded to hurricane status.

Iselle was about 620 miles (997 km) east of Hilo, on the Island of Hawaii, at 11 a.m. Hawaiian Standard Time on Wednesday and heading west-northwest at 15 miles per hour (24 km per hour) with maximum sustained winds of 90 mph (144 kph), according to the National Hurricane Center.

Iselle was expected to weaken into a tropical storm before reaching the islands on Thursday afternoon but forecasters said it could still bring high winds and 10-to-15-foot (3-to-4.5-meter) surf to the tourist haven.

State officials warned of the potential for flash floods, mudslides and power outages and Governor Neil Abercrombie signed an emergency proclamation, freeing up funds and other resources, in anticipation of its arrival.

"We want to make sure we are doing everything possible to protect the public," Abercrombie said in a statement released by his office. "This proclamation improves the state's ability to respond quickly to any potential impacts from both storms."

Meanwhile Hurricane Julio, upgraded on Wednesday from a tropical storm, was expected to roll into the islands as early as Saturday, also packing heavy surf and high winds. But officials said they were still focused on Iselle.

Residents stocked up on basics as authorities in Honolulu advised them to prepare a seven-day disaster supply kit. Shoppers waited in lines at supermarkets with carts full of bottled water, batteries and nonperishable food.

"With Hawaii's remoteness, it could be as long as a week before a full disaster relief operation can be initiated," the Honolulu Department of Emergency Management said in a statement.

Honolulu teacher Gina Nakahodo said she had remained calm until she reached the empty water aisle of her local grocery store early on Tuesday.

"We've had so many storms that have passed us by, but with these two back to back you begin to worry. Then all of a sudden the aisles are empty and there's no water and it makes your heart pound a little," Nakahodo said.

She said she talked to a couple visiting from California and told them everything was going to be OK. "But in the back of my mind I'm wondering, 'what's going to happen?'" she said.

The Coast Guard warned of heavy weather by Thursday, with the two hurricanes expected to generate extreme sea conditions, storm surge and surf of 10 to 15 feet throughout the island chain.

The National Weather Service issued a flash flood watch from early Thursday to early Saturday, with Hurricane Iselle expected to bring heavy rains to the islands.

Public schools would be closed on Thursday on the islands of Maui, Molokai, Lanai and the Island of Hawaii, the Hawaii State Department of Education said. (Reporting by Malia Mattoch McManus; Additional reporting by Eric M. Johnson in Seattle, Daniel Wallis in Denver and Dan Whitcomb in Los Angeles; Writing by Dan Whitcomb; Editing by Jeremy Gaunt, Mohammad Zargham and Eric Beech)